1. Field of the Invention
The present invention relates to a solid state imaging device, a driving method of the solid state imaging device, and electronic equipment.
2. Description of the Related Art
An example of a solid-state imaging device is a CMOS image sensor which reads out photo-generated charges accumulated in the p-n junction capacitance of photodiodes, which are photoelectric conversion devices, by way of MOS transistors. With such a CMOS image sensor, actions for reading out photo-generated charges accumulated in photodiodes are executed for each pixel, each line, or the like. Accordingly, the exposure period for accumulating photo-generating charge does not match for all pixels, and distortion occurs in the image when the subject is moving, etc.
FIG. 38 illustrates a structural example of a unit pixel (hereinafter may be referred to simply as “pixel”). As shown in FIG. 38, the unit pixel 100 is of a configuration including, in addition to a photodiode 101, also a transfer gate 102, an n-type floating diffusion (FD) 103, a reset transistor 104, an amplifying transistor 105, and a selecting transistor 106.
With this unit pixel 100, the photodiode 101 is an embedded photodiode wherein a p-type layer 113 has been formed on a p-type well layer 112 formed on an n-type substrate 111 and an n-type embedded layer 114 embedded. The transfer gate 102 transfers charges accumulated at the p-n junction of the photodiode 101 to the floating diffusion 103.
Mechanical Shutter Method
One widely-used way of realizing global exposure, which is an arrangement wherein imaging is performed with all pixels exposed at the same exposure period, with the solid state device having the above-described unit pixel 100, is the mechanical shutter method which uses mechanical light shielding. Exposure is started for all pixels at the same time, and exposure is ended for all pixels at the same time, whereby global exposure is realized.
With the mechanical shutter method, the period during which light is input to the photodiode 101 and photo-generated charge is generated is the same for all pixels, due to mechanically controlling the exposure time. With this system, the mechanical shutter closes so substantially no more photo-generated charges are generated, and in this state signals are sequentially read out. However, reduction in size is difficult since a mechanical shielding mechanism is used, and also the mechanical shutter method is inferior to electrical methods with regard to simultaneity as well, due to limitation of the mechanical driving speed.
Global Exposure According to the Related Art
Operations for realizing imaging with the exposure period matched for all pixels and with no distortion using the unit pixel 100 shown in FIG. 38 will be described with reference to the operation explanatory diagram in FIG. 39 and the timing chart in FIG. 40.
First, a discharging operation for emptying the accumulated charges in the embedded photodiodes 101 of all pixels at the same time is executed, and exposure is started (1). Thus, a photo-generated charge is accumulated at the p-n junction of the photodiode 101 (2). At the point that the exposure period has ended, the transfer gate 102 is turned on for all pixels at the same time, so all of the accumulated charges are transferred to the floating diffusion (capacitance) 103 (3). Closing the transfer gate 102 holds the photo-generated charges accumulated over the same exposure period for all pixels in the respective floating diffusions 103. Subsequently, the signal levels are sequentially read out to a vertical signal line 200 (4), following which the floating diffusion 103 is reset (5), and after this the reset level is read out to the vertical signal line 200 (6).
After having read out the signal level and reset level to the vertical signal line 200, noise removal processing of the signal level is performed using the reset level, in downstream signal processing. With this noise removal processing, the reset level of the reset operation executed after reading out the signal level is read out, so kTC noise occurring in the reset operation is not removed, which can lead to image deterioration.
The kTC noise occurring in the reset operation is random noise generated by the switching operations of the reset transistor 104 at the time of the reset operation, so the signal level noise will not be precisely removed unless using the level before transfer of the charge to the floating diffusion 103. The charge is transferred to the floating diffusion 103 at the same time for all pixels, so the reset operation is performed again following reading out the signal level, and noise removal is performed. Accordingly, noise such as offset error and the like can be removed, but this is not the case with kTC noise.
Now, we will refer the readout period of the signal level as “D period”, and the readout period of the reset level as “P period”. There are many crystal flaws at the Si—SiO2 interface, and dark current readily occurs. In the event of holding that charge at the floating diffusion 103, there is difference which occurs in the dark current applied to the signal level, depending on the readout order. This also is not cancelable with noise removal using the reset level.
Pixel Structure Having Memory Unit
One proposal which has been made as a way to deal with the problem of the above-described kTC noise not being removable is a unit pixel 300 which has a memory unit (MEM) 107 separately from the floating diffusion 103 within the pixel (e.g., see Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-502722 and Japanese Unexamined Patent Application Publication No. 2006-311515) as shown in FIG. 41. The memory unit 107 temporarily holds the photo-generated charge accumulated at the embedded photodiode 101. The unit pixel 300 further is provided with a transfer gate for transmitting the photo-generated charge accumulated at the photodiode 101 to the memory unit 107.
The operations for executing global exposure at the unit pixel 300 having the memory unit 107 will be described with reference to the operation explanatory diagram in FIG. 42.
First, a discharging operation is executed for all pixels at the same time, and exposure is started (1). A photo-generated charge is accumulated at the photodiode 101 (2). At the time of ending the exposure, the transfer gate 108 is driven for all pixels at the same time to transfer the photo-generated charge to the memory unit 107, where it is held (3). Following exposure, the reset level and signal level are read out in sequential operations.
First, the floating diffusion 103 is reset (4), and next the reset level is read out (5). Subsequently, the charge held at the memory unit 107 is transferred to the floating diffusion 103, and the signal level is read out (7). At this time, the reset noise included in the signal level matches the reset noise read out in the reset level readout, so noise reduction processing including the kTC noise as well can be performed.
As can be understood from the above description, a pixel structure having the memory unit 107 for temporarily holding the photo-generated charge accumulated at the embedded photodiode 101 can realize noise reduction processing including the kTC noise as well.